Liquid crystal display devices are presently widely used as display devices which can realize thin structure/light weight. The display characteristics of liquid crystal display devices are known to be substantially influenced by the alignment of liquid crystal, the pretilt angle of liquid crystal, the stability of the pretilt angle, electrical characteristics, etc. In order to improve the display characteristics of such liquid crystal display devices, not only the liquid crystal material to be used but also the liquid crystal alignment film which is in direct contact with the liquid crystal and which is decisive to its alignment state, becomes important.
At present, a liquid crystal alignment film is formed mainly by using a resin solution of a polyamic acid or a polyimide as a liquid crystal aligning agent in such a manner that such a liquid crystal aligning agent is applied to a substrate, followed by baking, and the surface of the coated film is subjected to so-called rubbing treatment i.e. rubbing the surface with a rayon or nylon cloth under pressure.
A method of obtaining a liquid crystal alignment film from a polyimide or a polyamic acid as its precursor, has been widely industrially employed and is still being used, since a coated film excellent in heat resistance and solvent resistance can be prepared by such a simple process as applying a resin solution followed by baking, and liquid crystal can easily be aligned by rubbing.
Further, in order to improve the display characteristics of liquid crystal display devices, various techniques have been proposed, such as variously selecting the structure of the polyamic acid or the polyimide or blending resins having different characteristics, to further improve the liquid crystal alignment, to improve control of the pretilt angle and its stability, to improve the voltage retention, to avoid accumulation of accumulation charge against a DC voltage or to improve efficient discharge of the accumulated charge. For example, JP-A-2-287324 proposes to employ a polyimide resin having specific repeating units in order to obtain a high voltage retention. Further, JP-A-10-104633 proposes to shorten the time till the residual image disappears, by using a soluble polyimide having a nitrogen atom in addition to the imide group, against the image retention phenomenon.
However, along with the progress in high performance of liquid crystal display devices, energy saving of display devices, improvement in durability in various environments, etc., problems have become distinct such that the contrast decreases since the voltage retention in a high temperature environment is low, and that image persistence is likely to result when a charge is accumulated by continuous driving for a long time, and it is difficult to solve such two problems simultaneously only by the techniques heretofore proposed.